This invention relates to the generation of synchronizing pulses timed to a particular portion of a waveform having a constant duty cycle over a continuously varying range of frequencies. In particular, it is useful to trigger an event to be photographed during the open period of the shutter of a high-speed framing camera during the period that the camera is accelerating as well as during periods which vary about its controlled speed.
The high-speed framing camera is a well-known device used to make photographs of events that occur very rapidly. Such cameras are commercially available with framing speeds as high as 10,000 pictures per second. In order to obtain light levels that permit adequate photography at such speeds, it is frequently desirable to obtain a trigger pulse synchronized with the camera that permits either the triggering of an event to be photographed or the triggering of a strobe lamp. When the framing camera has achieved a constant or nearly constant speed, synchronization is not a particularly difficult problem. A nominally rectangular wave is normally generated by either a magnetic or an optical pickoff associated with the shutter-driving mechanism. A magnetic pickoff may sense special paramagnetic or ferromagnetic screws in a rotating shutter, generating an output consisting of a rectangular wave form associated with passage of the magnetic screw past a coil. An optical pickoff is frequently used in which a light beam is chopped by a portion of a rotating shutter to generate a rectangular wave. In each case, the result after reshaping through a Schmitt trigger is a substantially rectangular wave of a fixed duty cycle at a frequency equal to the picture framing rate. If the camera is operating at a constant speed, synchronization is a simple matter of shaping the output pulse and delaying it to time a strobe lamp or a triggered event into the open period of the shutter.
The instant invention was developed to obtain photographs of information recorded on a hodoscope. The particular hodoscope in question included an array of 720 neon bulbs, some of which were to be switched on during the period when the camera shutter was open. These lamps are flashed on for 150 microseconds. The camera that was used was run at speeds up to 5000 frames per second, so that the corresponding period of the shutter cycle was 200 microseconds. In this camera, a shutter restricts light transmission to approximately 80 microseconds out of the 200 microseconds at 5000 frames per second. This duty cycle remained constant when the camera was being accelerated from a stop to the rate of 5000 frames per second, but the open period of the shutter was extended proportionately at lower speeds.
Attempts to use a constant delay from the camera shutter pulse to trigger the hodoscope in this application were marginally effective at high speeds and totally ineffective during the period of acceleration of the camera. It was found that film was sometimes inadequately exposed at fast framing rates. It was also found necessary to set the delay for each run and to make a test run prior to each run in order to be sure that the delay was proper for the speed being used. The test run required exposing a test film and developing the film to assure that the exposure was adequate.
It is an object of the present invention to provide a method and means of generating a synchronizing pulse that is located at a fixed relative position in a repetitive waveform.
It is a further object of the present invention to provide a method and means of generating a synchronizing pulse occurring during the same relative position in a repetitive wave form over a range of frequencies.
It is a further object of the present invention to provide a method and means of shaping the triggering output from a high-speed framing camera to generate a triggering pulse during the open period of the camera shutter.
It is a further object of the present invention to provide a method and means of generating a triggering pulse to permit the photographing of events with a high-speed framing camera during acceleration and constant-speed operation of the high-speed framing camera.
It is a further object of the present invention to provide a method and means to locate an event at a fixed fractional location in the period of a repetitive waveform by multiplying the time period of the repetitive waveform by a constant.
Other objects will become apparent in the course of a detailed description of the invention.